idle screw does not raise the idle speed

[older2smokers]

I put a PWK on a 95 Yamaha wr250z (replaced the orig Mikuni).  bike will start and run okay (depending on exact needle and jets) for about 10 minutes.  idle speed then drops way down and at that point the idle speed screw does not change the rpm.  the air screw (AS) will affect how quickly it returns to idle but doesn't change the idle speed.  bike will die if AS all the way closed or if out 4-5 turns.  slide is open a good 1/8" compared to my KDX the slide is barely open.

I've checked the jet block gasket (which was new), the float level, and the jetting/needle is at or close to the 95 yz sizes (they came stock w/pwk). also the carb is a Power Jet model but that is blocked off.  I also have a clone carb (ChiHin i call it) that does this same this.

I'm waiting for a new cable to re-install the Mikuni until I can get some insight on this issue.
thanks for any and all input.   

[m in sc]

sounds like the pilot isnt the right size or the fuel level is incorrect. sounds like the pilot is too small IMHO

[older2smokers]

not to say you are wrong but just to understand why-  if Pilot Jet (PJ) is wrong then why would it idle at correct speed at first?  then all of a sudden change?

float level being too low is something i considered as that would make sense but then wouldn't the idle go higher as it is being leaned out?  which it does do if close petcock while running (after a couple minutes).  not sure what it would do if level was high other leak out the overflows or run rich.  that wouldn't explain the sudden drop of rpm after warm-up. 

i appreciate the input.  i'm afraid i might be over thinking it OR something wrong downstream of the carb (which is why i will re-install the Mikuni).

[m in sc]

it wouldn't idle at all if the fuel level was too low. it has to be high enough to hit the pilot. that being said, the fact it needs the slide raised that much which will draw fuel from the needle circuit tells me it -might-  be too lean.  thinking about it a bit more..it could Aldo be fuel starvation... in regards to feed. if the petcock or tank vent is clogged, it may not be able  to refill the bowl in 10 Mins of idling.  try to idle it w the gas cap off the bike. if it still dies, then put cap back on, check flow of petcock.  for, a minute into a bucket . watch it. if it decreases pretty quick, might be that.if all that checks out.. might be electrical but, thats a different ball game